The present invention relates in general to power converters and, more specifically, to a full bridge DC/DC converter having an improved ZVS (Zero Voltage Switching) operation mode and primary winding clamp circuit to minimize secondary reverse recovery current losses.
A common practice in the field of power conversion is to use switching power supplies to convert DC voltage of one level to an isolated DC voltage at a second level. A circuit topology that is well suited for this purpose is the full bridge converter. One of the major disadvantages to switching power conversion is the losses associated with the switching elements in the power converter, and a common approach is to utilize nearly zero voltage switching (ZVS) to minimize these losses.
A common ZVS topology for a prior art full bridge converter is a phase-shifted full bridge. Such a circuit is described in detail in a Texas Instruments (formerly Unitrode Corp.) generated application note U-136A entitled xe2x80x9cPhase Shifted Zero Voltage Transition Design Considerations and the UC3875 PWM Controller,xe2x80x9d published in May 1997 and presently available from Texas Instruments, Inc. The phase shifted full bridge described therein relies on the parasitic elements of the switching elements, typically MOSFET transistors, and transformer primary winding inductance to transition the voltage across the switching elements to zero prior to turning on these switches. Since the switching losses are a function of the voltage across the switch prior to turn on, this approach reduces these losses to near zero under most operating conditions or characteristics.
While the phase-shifted full bridge minimizes switching losses associated with the primary circuit elements, it does not address inherent switching losses caused by output rectifier diode reverse recovery. These losses are primarily associated with the reverse recovery time of the secondary diodes, and result in ringing and emi (electromagnetic interference) when combined with parasitic elements of the main power transformer. Common prior art approaches to mitigate these losses have included using one or more of dissipative snubbers, saturable reactors, primary clamping circuits and low loss active filters.
By reducing the rate of change of current in the output rectifiers, the peak reverse recovery current is limited in some of the prior art circuits. Clamping elements on the primary side of the power train in other of those circuits have resulted in the capture of a majority of the reverse recovery energy in the primary resonant inductors, thus minimizing the dissipated energy.
Although these attempts to mitigate losses have, to various degrees, improved power dissipation efficiency from circuits not using the described approaches, there are still drawbacks of existing known circuits. These include, among others, cost, control, excessive emi and excessive switching losses.
It would thus be desirable to provide a converter with increased power conversion efficiency whereby a given size converter container can provide a larger amount of output power, or a given power rated converter can be packaged in a smaller container. It would also be desirable to provide a converter that minimizes component stresses and reduces generated high frequency interference signals, such as emi, for example.
The present invention comprises a full bridge non-resonant pulse-width-modulated (PWM) switching circuit. The circuit comprises four switches, inductive device(s), clamp diodes, a transformer and an output rectification stage including a filter connected to a transformer secondary. Capacitor and diode elements associated with the switching elements (part of the switching devices or external) are operatively connected to the four switches so as to switch current through the switches at substantially zero voltage (ZVS). The inductive device(s) and clamp diodes reduce the primary transition shoot through current resulting from reverse recovery of the transformer secondary rectification stage. The optional use of a current doubler as part of the filter circuit operates to reduce conduction losses in the main power transformer whereby even greater power conversion efficiency is obtained.